Water Level Sensor Circuit

Thread Starter

JulesP

Joined Dec 7, 2018
265
Hi all,

I have built a water level feed control for an electrolyser using the attached circuit but it doesn't quite work as I would like it to.

The sensor consists of two Allen screws in the side of the electrolyser body (test rig) acting as the sensor and when the water level falls below the top screw the motor starts up as expected. However, when the water level rises back to the top screw the motor doesn't switch off.

There is a 10k pot in the circuit that doesn't seem to make any difference to how the device works and what I'm asking is how can I make the motor stop immediately once the water (0.05M KOH) rises to the top screw.

I'm thinking that perhaps the 330uF cap is holding the base of the first transistor on for too long? So maybe a smaller value like 50uF or a 1k bleed resistor across it?

Thoughts appreciated.

Jules
 

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Thread Starter

JulesP

Joined Dec 7, 2018
265
hi,

Update:
Ignore the above I see they have linked Q1E and Q2E in copper.!



Added:
Is this a dry joint.??
I made the PCB based on the circuit and yes there is copper pour for the GND. I think there is a good resistor connection on the reverse side which is where I apply the solder.
 

AlbertHall

Joined Jun 4, 2014
10,226
What is the resistance between the bolts when they are covered by liquid?
I make the trip points of the schmidt trigger 4V and 5.5V which seems reasonable.
 

Thread Starter

JulesP

Joined Dec 7, 2018
265
What is the resistance between the bolts when they are covered by liquid?
I make the trip points of the schmidt trigger 4V and 5.5V which seems reasonable.
I will look later but a standard meter was wobbling all over the place in the 6-10MOhm range so I put my little tester on it so I’ll look those up. From memory when the pins are covered it said 127uF and ESR 0.15k.
 

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AlbertHall

Joined Jun 4, 2014
10,226
That shouldn't upset your circuit too much but it will probably mess up resistance readings.
With it connected to your circuit, measure the voltage between the screws with and without liquid.
 

Thread Starter

JulesP

Joined Dec 7, 2018
265
That shouldn't upset your circuit too much but it will probably mess up resistance readings.
With it connected to your circuit, measure the voltage between the screws with and without liquid.
The readings are 1.4mV with liquid over both studs and 0V with no liquid.
 

Thread Starter

JulesP

Joined Dec 7, 2018
265
No power to circuit. You didn’t mention having it powered up. I’ll redo measurements in a while if you need it powered on
 
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